Am7 is a four-note chord consisting of the notes A, C, E, and G, and following the minor seventh formula of 1 b3 5 b7. An Am7 voicing in open position would look like this:
-0-
-1-
-0-
-2-
-0-
---
This differs from an A minor triad by the addition of the minor seventh, here played on the open G string.
The following are moveable Am7 voicings, meaning they can be played in different places on the fretboard to produce different minor seventh chords, as they contain no open strings:
-3---5---8-----------------------------
-1---5---8---5-------------13----------
-2---5---9---x---9----12---12----------
-2---5---7---5---7----10---14----------
-------------3---10---12---12----------
-------------5---8----12---------------
To play any given minor seventh chord, slide these fingerings up or down until you reach your intended destination. For example, C is three half-steps above A, so moving any of these voicings up by three frets will produce a Cm7 chord. To increase your fretboard familiarity, try finding your own voicings. Simply locate the four chord tones anywhere on the fretboard and strum away!